Spray drying and agglomerating are well-known unit processes that have been employed in the food processing industry for some time. Generally, however, spray drying and agglomerating are separate processes which are carried out in stepwise fashion on coffee extracts, liquid milks and the like. It is apparent that such separate stepwise processes have a detrimental effect on the flavor quality of heat sensitive food products and leave much to be desired in terms of plant economies, utilization of equipment and throughput times. It would be desirous, therefore, if a combined spray drying and agglomerating process were able to be devised so as to increase the production capabilities of a food processing plant and at the same time to reduce the amount of equipment and throughput times that might otherwise be required.
The concept of a combined spray-drying/agglomerating process has heretofore been described in several U.S. patents, namely, U.S. Pat. No. 3,514,300 to Mishkin et al. and U.S. Pat. No. 3,151,984 to Peebles et al. In the Mishkin et al. process, recycled fines are employed in the production of an agglomerated soluble coffee powder. In the process described by Peebles et al., milk concentrate with added lactose crystals is introduced into a spray dryer and with proper control of drying conditions is discharged as an aggregated material having a moisture content of 10% to 20%. A second drying operation is then employed in the Peebles et al. process to further reduce the moisture content. In both the Mishkin et al. and Peebles et al. processes, however, it appears that the introduction of dry solids, i.e., recycled coffee fines and lactose crystals is essential for production of the desired end product.
The concept of foaming coffee extracts and other extracts and suspensions of food products to control final product color, density and particle size has also been described heretofore. Thus, U.S. Pat. No. 2,788,276 to Reich et al. teaches a process for spray-drying a foamed material such as coffee. However, the objective referred to in the patent is that of producing a product with discrete spherical structures which are not clumped, aggregated or otherwise agglomerated.
It is well known in the art of spray drying that, for a particular spray nozzle system, optimum atomization occurs when the liquid flow rate exiting from the nozzle is of sufficient magnitude to cause the nozzle to deliver the spray over a wide area in the form of small droplets (fine spray). Conventionally, high pressure positive displacement pumps are employed to force the liquid through the spray nozzle at the desired, substanitally uniform, flow rate to form a spray pattern which conforms to the structural dimensions of the drying tower and which has drop size uniformity which conforms to the drying capability of the tower.
As is well known, reciprocating positive displacement pumps develop a fluctuating pressure and fluctuating flow rate of discharge liquid. The fluctuating flow rate of liquid through the spray nozzle creates fluctuations in the spray pattern (conical angle of discharge) and also non-uniformity of drop size in the spray. For the most part, in conventional spray drying practice, the magnitude of these fluctuations is minimal and creates a change in conical spray angle of about 3.degree.-4.degree.. Conventionally, operations are conducted with systems which develop substantially uniform flow rates. In many instances, a gear-type positive displacement or multi-piston pump is employed to assure the desired uniformity of flow rate at the high pressures. Also, in many installations a dome-type accumulator is located between the pump discharge and the spray nozzle to even out the flow rate to the nozzle.
For some liquids, namely cottage cheese whey, fluctuations in flow rate through the spray nozzle (commonly called "slugging") when using a high pressure recprocating pump has been minimized by the injection of gas into the whey in the system between the pump and the spray nozzle (cf. Hanrahan, U.S. pat. No. 3,222,193).
For a particular spray drying system, the flow rate through the spray nozzle should be of a value for the nozzle to deliver a spray of desired atomization without endangering the operation by creating too wide a spray angle and thus wetting the side walls of the drying tower. Too low a flow rate will create little or no break-up of the liquid into drops or will develop drops of such large size they will be incompletely dried in the tower.